1. Field of the Invention
The present invention relates to a mobile communication system and a method of controlling a location registration processing in the mobile communication system and, more particularly, is directed to a mobile communication system in which the service area is composed of small radio zones and directed to a mobile communication system supposed to use at low speed or in stationary conditions, such as a portable radio telephone system in which a location registration processing of a mobile telephone system is required.
2. Description of the Prior Art
In conventional mobile communication systems such as a mobile telephone or the like in which a plurality of zones are combined to constitute a service area, in order to reduce unnecessary calls when a mobile station receives a call from a base station, there is adopted such a system in which a plurality of calling areas, each composed of a plurality of zones, are provided within the service area and a mobile station is accessed from a base station located within a calling area whose location is registered.
In general, when a mobile station is powered or enters the service area from the outside of the service area, a location registration processing is carried out. A book that had been edited by Masaaki Shinshi and published on September 1989 under the title of "Mobile Communication", for example, describes these location registration systems.
As an example of the location registration system, there are known a system in which a base station receives a signal from a mobile station and the base station or a radio exchange station judges the mobile station's location in the service area and a system in which the mobile station receives a signal from the base station to judge its own location in the service area.
FIG. 1 of the accompanying drawings shows in functional block form an example of the conventional location registration system in which the mobile station judges its own location in service area.
As shown in FIG. 1, the conventional mobile station includes a radio unit 1 for effecting the transmission or reception between it and the base station (not shown) via a radio signal through an antenna 1A, a channel quality measuring apparatus 4 for receiving a received signal (control signal) from the base station to evaluate the channel quality, a data identifying apparatus 3 for identifying from a received signal a calling area name (described later on) or a location registration area name (described latex on) and a base station name (i.e., an ID (identification) code of the base station) or a zone name (ID code when zones are composed of a plurality of base stations), a registration information memory 5 for storing therein a location registration information that was already registered, a comparing and judging apparatus 6 for comparing an information concerning the base station having a best channel quality from the channel quality measuring apparatus 4 and the data identifying apparatus 3 with the content of the registered location registration information stored in the registration information memory 5 and a location registration processor 2.
In the above description, the calling represents operation such that, a plurality of zones are grouped and, a plurality of base stations in the same group call a particular mobile station at the same time according to some suitable methods, such as a TDMA (time division multiple access) method or the like. Further, the calling area names represent ID code of groups of a plurality of zones (or base stations). The location registration area name is as follows. In general, since the service area is composed of a plurality of calling areas, when calling a particular mobile station, requiring information of present location of each mobile station. The location registration is the operation of registering a present location of each mobile and, the location registration area is composed of a plurality of zones (or base stations). The location registration area names represent the ID code representing these groups of a plurality of zones. Sometimes the calling areas are the same the location registration areas.
A location registration processing of the conventional system shown in FIG. 1 will hereinafter be described with reference to a flowchart in FIG. 2.
As shown in FIG. 2 of the accompanying drawings, when the mobile station executes the location registration processing, in decision step S1, the mobile station receives a signal of a frequency band that was allocated in advance and searches a base station that the mobile station can communicate with. In decision step S1, the base station that can be communicated with the mobile station is retrieved by the channel quality measuring apparatus 4 and the data identifying apparatus 3. If the base station that can be communicated with the mobile station is not searched as represented by a NO at decision step S1 because the level of the received signal is low or the like, then the processing proceeds to step S2, whereat a message representative of "out of service area" is displayed. Then, the processing returns to decision step S1, whereat the base station is searched at a preset time interval. If the base station that can be communicated with the mobile station is searched as represented by a YES at decision step S1, then the processing proceeds to step S3, whereat the data identifying apparatus 3 reads out the location registration area name or the calling area name, the base station name or the zone name from the received signal and the channel quality measuring apparatus 4 evaluates the channel quality between the base station and the mobile station on the basis of a received electric power or error rate of received data. Then, a base station whose channel quality is regarded to be best is selected from a plurality of base stations that can be communicated with the mobile station by the comparison of a received electric power, an error rate of received data or the like, and the location in service area is judged on the basis of an information concerning the base station thus selected.
Then, the processing proceeds to the next step S4, whereat the last location registration information and the location in service area are compared with each other. If the location in service area is different from the last location registration information, then a signal is transmitted to the selected base station that can be communicated with the mobile station to thereby effect the location registration processing in step S5. The base station receives necessary information such as mobile station name or the like in response to the request of the mobile station and registers the same in a memory station that administers a location information of each mobile station.
Referring back to FIG. 1, the comparing and judging apparatus 6 judges optimum base station and location in service area, and compares the location in service area with the last location registration information stored in the registration information memory 5 to thereby determine whether or not the location registration is to be executed. After the location registration processing at step S5 was finished, the mobile station enters the intermitting reception state in order to reduce an amount of consumed electric power so that it may receive an information from the base station at a constant time interval to await the call from the base station. If the mobile station receives a call request from the base station as represented by a YES at decision step S6, then the mobile station executes the call processing at step S7. During the intermitting reception state, or if the call request is not received as represented by a NO at decision step S6, then the processing proceeds to decision step S8, whereat the evaluation of the channel quality is effected by the channel quality measuring apparatus 4. If it is determined in decision step S8 that the channel quality of the base station is not satisfactory or that the reception becomes difficult such as when the reception level of the base station is lowered due to the motion of the mobile station, then the processing returns to the decision step S1, whereat the base station that can be communicated with the mobile station is again searched. If the base station name or the zone name, the location registration area name or the calling area name is changed from a received signal of a new base station supplemented, then the location registration processing is executed one more time at step S5.
The location registration area or the calling area is generally constructed by the combination of a plurality of zones. Accordingly, if the location registration area or the calling area is not changed even though the base station that can be communicated with the mobile station is changed due to the motion of the mobile station or the like, the location registration processing need not be executed one more time.
In general, a moving speed of a mobile station is changed a lot in accordance with the situation under which the user of the mobile station is placed. If the user of the mobile station is in a high speed vehicle such as a train, a bus or the like, then a moving speed of the base station is increased about ten times or more as compared with the moving speed at which the user having the mobile station is walking. In order to call the mobile station, the location registration must be updated each time the location registration area or the calling area is changed in accordance with the motion of the mobile station. Therefore, during the mobile station is in motion at high speed, a frequency at which the location registration is updated is increased about 10 times or more as compared with a frequency at which the location registration is updated during the mobile station user is walking. That is, according to the conventional location registration system, the mobile station within the high speed vehicle must frequently effect the location registration so that a location registration control unit in a system having a number of mobile stations accommodated therein suffers from an increased burden.
Further, there is the large possibility that, even when the base station sends a call request to the mobile station located within the high speed vehicle on the basis of the location registration information, the mobile station cannot answer the call request from the base station because the mobile station is moved to another calling area or to the outside of the service area and also the location registration information is not yet registered again, which as a result increases a blocking probability. In order to reduce a blocking probability, it is proposed to set a wide calling area. Alternatively, there is proposed a method such that, when the mobile station cannot answer the call request from the base station in the calling area whose location is registered, a call request is executed by the calling area of the surroundings. If these methods are realized, then the mobile communication system unavoidably suffers from an increased burden with respect to the location registration processing and the call processing from a system control standpoint.
If the location registration processing, frequently executed by the mobile station that is in motion at high speed., is controlled from a services standpoint, then an amount in which a burden imposed upon the system is increased due to the concentration of a lot of unnecessary location registration requests can be alleviated or the system can be simplified. Further, if the occurrence of the unnecessary location registration requests is alleviated, then a frequency at which the mobile station effects the transmission for location registration processing can-be reduced and the amount of the electric power consumption of the mobile station can be reduced effectively.
To restrict the location registration processing executed by the mobile station that is in motion at high speed, the moving speed of the mobile station must be understood. In on-vehicle radio transmitter and receivers, typically represented by the mobile telephone system, the moving speed of the mobile station can be constantly measured by utilizing a speedometer. However, it is difficult to measure the moving speed of the mobile station while the user of the mobile station is on the high speed vehicle such as a bus or the like.